WESTBORO (CBS) – A woman was killed on Friday when a backhoe being towed by a National Grid dump truck rolled off the trailer and landed on a minivan carrying four people.

That woman, who was in her seventies, was a passenger in the back seat of the car. She was pronounced dead at the scene.

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The driver, 42-year-old Yingzi Wang from Westford, was flown to UMass Medical Center once emergency responders were able to free her from the car. Police say two children, a 10-year-old boy and a 13-year-old girl were taken to the hospital by ambulance.

WBZ-TV’s Jim Armstrong reports

According to State Police, the driver of the dump truck that was carrying the backhoe is 49-year-old Jeffrey Fisher, a construction supervisor for National Grid who lives in Coventry, Rhode Island.

Jeff Munson was driving his wife and two children to the Cape, when he saw the crash.  He says it looked like the driver hauling a backhoe on a flatbed was trying to switch lanes, when he fishtailed.

“It jackknifed and rolled and he went to go to the right lane, he over-compensated and the backhoe fell off the trailer,” said Munson.

Munson rushed to help.  “I saw a boy all bloody, covered in bloody, but he was coherent,” he said. Munson also told WBZ-TV that the boys mother and a young girl were both conscious.

The scene of the crash was south of the Simarano Drive exit in Southborough, right on the Westboro line.

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The two right travel lanes and the breakdown lane were closed for approximately six hours and traffic was delayed for more than three miles. All lanes were opened at 8:00 p.m.

Whether the backhoe was properly secured on the trailer was not known right away.  Police told WBZ-TV that they had not ruled out the possibility of charges.

Friday evening, National Grid issued a statement expressing sympathy and concern for the victims.

The company says it’s launching an internal investigation and cooperating with police.

In the statement, National Grid also offered information that hints to a possible cause of the accident, saying the truck “was being test-driven at the time of the accident. This was a new model of dump truck which National Grid was continuing to evaluate in test drives following some safety considerations which had been identified by our employees.”

State Police say the truck involved was a 2010 Freightliner M2106.

It was unclear whether those concerns played any role in the accident.

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WBZ-TV’s Karen Anderson contributed to this report.